Method of dissolution of metals using thermogalvanic cells

ABSTRACT

Thermogalvanic cells comprising two half cell units each containing an inert electrode, e.g., graphite the half cells being connected at their upper and lower portions by conduit means; cell adapted to dissolution of metals, e.g. stainless steel - in operation, electrode and metal to be treated charged to each half cell and heat applied to first half cell, continuous convection circulation of electrolyte as dissolution occurs.

United States Patent Decrosta Nov. 20, 1973 [54] METHOD OF DISSOLUTIONOF METALS 1,160,400 11/1915 Goldschmidt..... 204/146 USINGTHERMOGALVANIC CELLS 3,198,720 8/1965 Knippers et al... 204/1463,194,750 7/1965 Knippers et al... 204/146 6] Inventor: Edward Decrosta,7 James 1,511,967 10 1924 Holland 204 146 Hudson, N.Y

[22] Filed; Nov, 2, 1970 Primary Examiner.lohn H. Mack AssistantExaminerT Tufariello [21] Appl' 86318 AttorneyRobert A. BurroughsRelated US. Application Data [62] l3)i5v;s7io9n7; f Ser. No. 607,241,Jan. 4, 1967, Pat. No. 57 ABSTRACT Y I Thermogalvanic cells comprisingtwo half cell units 52 US. Cl. 204/146, 204 249 each Containing an inertelectrode, -ggraphite the 511 1111.01 B0lk 1/00, BOlk 3/03 half cellsbeing Connected at their upper and lower [58] Field of Search 204/146,248, 140, PortionS by conduit means; adapted to dissolution 204/249 ofmetals, e.g. stainless steel in operation, electrode and metal to betreated charged to each half cell and [56] References Cited heat appliedto first half cell, continuous convection UNITED STATES PATENTScirculation of electrolyte as dissolution occurs.

718,927 1 1903 Gould 204/248 8 Claims, 1 Drawing Figure METHOD OFDISSOLUTION OF METALS USING THERMOGALVANIC CELLS This application is adivision of my copending application, Serial No. 607,241 filed January4, 1967 and now Patent No. 3,537,972.

The present invention relates in general to thermogalvanic cells andmore particularly to thermogalvanic cells capable of providing anexceptional efficiency level of energy utilization, said cells beinguniquely and beneficially adapted for use in connection with a widevariety of industrial operations such as metals reclaiming and salvaginggas generation, the production of electricity, monitoring techniques andthe like.

As will be appreciated, a significant number of commercial activitiesdepend critically for feasible and economic practice upon theavailability of low cost energy sources, the diversion of such energy touseful purposes requiring, of necessity, relatively specific means. Inthis regard, exemplary reference may be made to the metals reclamationindustries wherein metal dissolution comprises a vital phase of theprocessing. Although the industrial applications involving theimplementation of methods designed to effect metal dissolution arelegion those peculiar to the basic metal processing industries inparticular have assumed a position of premier importance. Metaldissolution, in some form, is, of course, a necessary adjunct to a vastnumber of commercial operations associated with metal reclaiming,salvaging, polishing, as well as techniques evolved for purposes ofreducing a metal to its component constituents, i.e., the extraction ofcomponent metals from parent alloys and the like. The extent of metaldissolution will depend primarily on the nature of the operation; thus,in the case metal polishing or other operations involving the removal ofspurious coatings or other surface deposits, the dissolution effectswith correspondingly be confined to the surface portions of the metalarticle in question. In general, the techniques heretofore promulgatedfor such purposes and based upon the use of various forms ofmechanicalabrasion, e.g., rubbing, griiiding, etc., have proved notablydeficient and have thus necessitated resort to more palatable. chemicalmeans, the latter techniques invariably being based upon the utilizationof solvent media, e.g., acid baths to accomplish the requisite degree ofmetal dissolution. As will be recognized, with methods of the foregoingtype it is only necessary that dissolution be carried out to apredetermined depth of the metal article being treated.

The technology surrounding the efficacious practice of metal dissolutiontechniques has perhaps assumed paramount industrial importance inconnection with metal extraction, salvaging, reclamation, and similaroperations. The term metal extraction" as used herein is intended toconnote those unit processes and operations which may be resorted to forpurposes of reclaiming one or more constituent metals of a' parentalloy; thus, metal dissolution as an operative technique in this regardsignifies conversion of the base material to a form which permitsexpeditious separation, isolation and recovery of the various metalingredients. The burgeoning industrial importance of metal dissolutionmethodology as a means for achieving metal reclamation is, of course,well established and may be efficaciously employed in the treatment ofscrap materials as well as native ores.

Despite the relatively widespread industrial exploitation of metaldissolving processing to accomplish the aforestated objectives, seriousdisadvantages have nevertheless been encountered in its practice whichin the main have tended to retardor otherwise abate even furthercommercialization. In many instances, such processing provides butmarginal advantage and especially when considered from an economicstandpoint. Thus, even slight variations in market value of the metalsought to be reclaimed may militate against the propriety of a givenoperative technique. In fact, it may well be necessary to discard aconsiderable portion of the raw material metal supply should the metalcomposition,'i.e., content of valuable metal, fail to justify the costincrement incurred as a concomitant of the required processing. As apractical matter, the prohibitive economics involved has lead to vaststock pilings of raw material reserves considered economically unworthyof metal extraction treatment. The minimization of the cost factor ateach and every step of metals extraction processing is quite obviously amatter of critical import.

Considerable effort has thus been expended by skilled investigators andpractioners pursuant to the obtention of means whereby metals-extractionprocessing could be beneficially implemented with respect to rawmaterial metals containing but minimal quantities of valuable metalingredients. Despite the meritorious achievement which has typifiedsuchinvestigation, the economic problems continue to persist and haveheretofore represented a seemingly insurmountable challenge to metalsreclamation technology. The situation has been particularlyproblematical with respect to the reclamation of nickel from stainlesssteel alloys. It is a matter of common knowledge that nickel lendsitself admirably to a wide variety of commercial uses andparticularly'with reference to catalysis, e.g., the polymerization oforganic monomeric materials such as the alpha, beta-ethylenicallyunsaturatedmaterials commonly referred to as vinyl monomers; alkylation,isomerization, cracking and the like. In view of the commercialimportance of nickel, its efficient reclamation has become a matter ofprimary industrial importance. The

foregoing represents, of course, but one .particularized example of theurgency characterizing metals reclamation andmay well be extended toother metals including the reclamation of copper, manganese, tin fromtinplate, etc.

The methods heretofore evolved which purportedly enable theattainment-of metal extraction, dissolution,

etc., at least insofar as describedin the published literature bothpatent and otherwise, usually involves some type of electrolysistreatment wherein the metal to be treated is subjected to electrolysisin an appropriate electrolyte. Typically, the vast majority of suchmethods invariably require the utilization of a direct current sourceindependent of the energy generated during the course of the metaldissolution reaction. Although the costs attributable to the necessityof furnishing electrical energy may appear to be somewhat minimal in theagglomerate, it should be realized that the total profitloss picturecharacterizing processes of this type is highly tenuous and it is thusincumbent upon the processor to mitigate such expenses to the greatestextent possible. Further refinements and ramifications of the basicelectrolysis process have included various means for accelerating orotherwise expediting the metal dissolution reaction. For example, theutilization of heat, e.g., steam sparge, introduced directly into theelectrolyte medium is pretty much standarized procedure and provides asatisfactory measure of improvement as regard augmenting the metalelectrolysis reaction. However, implementation of the aforedescribedreactionaccelerating embodiments has tended to severely restrict theprocessor in his scope of operation, i.e., imposed rather stringentlimitations as regards the choice of reaction parameters includingtemperature, quantity of reactants and the like. Thus, in order toassure optimum realization of any possible advantage peculiar to suchtechniques, it has been imperative in practice to employ highly elevatedtemperatures, i.e., temperatures which materially promote electrolytecorrosivity, as well as other deleterious effects and thus makemandatory the use of specific and relatively costly materials ofconstruction. Moreover, since the electrolyte media is in many instancescomprised of highly volatile acidic substances having relatively lowsalt concentrations and thus exhibit comparactively high vapor pressuresat ambient temperatures, the useof closed systems for carrying out theelectrolysis reaction correspondingly requires the employment ofcontainer materials structurally adapted to Withstand any pressurebuild-up which might accompany the reaction. Other disadvantages whichhave tended to detract considerably from the commercial feasibility ofthe metal extractiontechniques heretofore promulgated, in the artinclude the requirement for using exceedingly high-energy electrolytemedia, i.e., solutions containing an inordinately high concentration ofelectrolyte. The use of such high concentration baths tends to aggravateproblems of the type previously stated and in some cases imposesintolerable requirements on the processor as regards the selection ofparameters for conducting the electrolysis reaction in question.

Thus, it is not surprising to find even today that many of the metaldissolution techniques practiced on a commercial scale, albeit somewhatmore sophisticated, nevertheless constitute ramifications of the basic,nonelectrolytic acid-treatment process, e.g., the dissolution of copperscrap in sulfuric acid at elevated temperatures, the latter beingeffected by the use of a steam sparge, for example. The latter hasproved to be a somewhat effective method since it combinesthereaction-accelerating effects resulting from the influence of highertemperatures with those attributable to agitation effects which ofcourse, are promotive of more efficient acid-metal contacting. Althoughother metal dissolution systems have been proposed which ostensiblycapitalize on the advantageous features inherent in both electrolyticand non-electrolytic processes, such systems are uniformly characterizedin that successful operation invariably requires materials ofconstruction, operating parameters or specific modes of operation whichmilitate against feasible commercial practice.

It is thus manifestly clear that there exists in the art a distinct needfor an efficient energy-producing cell beneficially adapted for use inconnection with the broad spectrum of operations incident to, forexample, metals extraction, metals reclaiming, as well as otheroperations and wherein the energy requirements for efficacious operationare reduced significantly.

In accordance with the discovery forming the basis of the presentinvention it has been ascertained that the basic principle governing theoperation of thermogalvanic cells as distinguished from cellsprincipally electrolytic in nature, can be synergistically modified toadvantage to provide a cell of novel construction and operation, suchcell being uniquely and beneficially adapted to a plurality ofindustrialapplications and particularly those relating to metals dissolutionprocessing.

The construction and operation of thermogalvanic cells is well known inthe art, being extensively described in the published literature bothpatent and otherwise. Perhaps the primary utility of cells of this typerelates to the production of electrical energy. Thus, such cells, eachcell being composed in turn of two half-cells, can be provided inbattery form utilizing series or series-parallel arrangements to makepossible the production of useful quantities of electrical energy. Inessence, thermogalvanic cells operate on the principle of mass fluxagainst a temperature gradient which in turn gives rise to the formationof a concentration difference across each of the half cells comprisingthe basic cell unit. The concentration differences thus formed lead tothe generation of an electromotive force and the consequent flow ofelectricity in a first or primary electric circuit. lons generated anddischarged at each of the electrodes contained in the respective halfcells are transported between the half cells to form an internal orsecondary electric circuit. Thus, with oxidation occurring at the anode,electrons are given up by a substance which becomes ionized, andsolvated in an electrolyte medium which can be solution, a molten salt,or, an ionized gas molecule. The electrons flow in the external circuitto the cathode, where positively charged ions recombine with theelectrons to form electrically neutral materials. Under the influence ofa temperature-induced concentration gradiant, diffusion of theelectrically-neutral species occurs, and the process is repeated.

Each of the electrodes employed in the aforedescribed cell structure isof course of the non-consumable type, i.e., undergoes substantially nonet change in mass. It is recognized that some loss in electrode massmay possibly be experienced due to what can perhaps best be described aswear and tear; consequently some degree of electrode dissolution, thoughminimal, may inevitably occur. It is axiomatic, however, that efficientthermogalvanic cell operation requires the observance of proceduresdesigned to minimize electrode loss and especially that which mightotherwise occur through dissolution. To this extent, then, theelectrodes can be regarded as permanent.

As sources of electrical energy, thermogalvanic cells are commonlyrecognized as providing a satisfactory level of operational efficienty.Heretofore, the principal of operation characterizing such devices hasbeen confined for the most part to the production of electrical energy.As will be made apparent in the discussion which follows, thermogalvaniccells fabricated in accordance with the present invention not onlyexhibit a manifold increase in operational efficiency, i.e., the totalenergy requirements for effective operation are reduced to an extentheretofore considered unattainable, but, in addition, due to novelaspects of construction, are peculiarly adapted to a mode of operationwhich greatly extends their field of use.

Thus, a primary object of the present invention resides in the provisionof a thermogalvanic cell wherein the foregoing and related disadvantagesare eliminated or at least mitigated to a substantial degree.

Another object of the present invention resides in the provision of athermogalvanic cell characterized by an exceptional efficiency level ofenergy utilization.

A further object of the present invention resides in the provision of athermogalvanic cell which affords exceptional means for effectuating therapid dissolution of metals with the use of but moderate conditions oftemperature, electrolyte concentration and the like.

A still further object of the present invention resides in the provisionofa thermogalvanic cell which is highly economical in construction andoperation to thereby make possible the realization of substantialsavings.

Other objects and advantages of'the present invention will become moreapparent from the following description and accompanying drawings.

The attainment of the foregoing and related objects is made possible inaccordance with the present invention, which in its broader aspectsincludes the provision of a novel thermogalvanic cell comprising asessential components a first half cell and a second half cell each ofsaid half cells comprising container means having disposed therewithinan inert unreactive electrode, a first conduit means interconnecting theupper regions of each of said container means, a second conduit meansinterconnecting the lower regions of said container means and whereineach of said conduit means communicates with the internal portions ofsaid container means.

The particular manner in which the improvements described herein may berealized can be perhaps best illustrated by reference to theaccompanying drawing which illustrates schematically the uniquearrangement of parts comprising the thermogalvanic cell structure. Eachof the half cells constituting the basic cell unit is representedgenerally at 1 and 2. Conduit means interconnection the upper and lowerregions of each of the half cells and serving as electrolyte bridges arerepresented at 3 and 4, respectively. lnert unreactive' electrodes 5 and5', e.g., graphite, are situated in such manner that at least a portionthereof protrudes internally of the half cells. The sole criticalitywith regard to the exact positioning of the respective inert electrodemembers 5 and 5' is such that it permits the establishment andmaintenance of adequate electrical contact therebetween via a suitableelectrically conductive path represented at 8 and coupled through switch7, the latter permitting insertion of voltmeter or ammeter A into theconductive path i.e., primary circuit 8. Thus, as illustrated in thedrawing, a portion of such electrodes projects externally of the halfcell housings, this being generally preferred since it affords readyaccess to at least two essential points of electrical contact whichserves to expedite any troubleshooting operaelectrodes, owing to theirsolubility in the acidic electrolyte media which during cell operationwould fill the respective half cells 1 and 2, can be characterized asconsumable, as distinguished from inert electrodes 5 and 5' which, ofcourse, are substantially immune to the dissolution effects of theelectrolyte. Each of half cells 1 and 2 may be further provided withsuitable wells, (not shown), to accommodate thermometers represented byT to permit necessary temperature readings. vent lines 29 and 30provided with valves 24 and 28 provide means for allowing gases toescape to the atmosphere during acid introduction.

l-lalf cells 1 and 2, via conduit lines 9 and 10 communicate withcontainer means 11, the latter serving as a gas liquid trap and allowsfor the separation of entrained electrolyte from the gases evolved fromeach of the half cells. Conduit line 13 interconnecting trap 11 and gascollecting tank 12 is provided with valve means 14 and permits removalof gaseous material evolved in the system. Entrained electrolyte may bereturned to the lower electrolyte bridge line 4 through line 15 providedwith valve means 16. Half cell communicates with acid reservoir tank,17thru line 27 provided with valve means 18. The upper electrolyte bridge,i.e., conduit line 3, is provided with valve means 19 to permit eithercontinuous or intermittent withdrawal of product solution, i.e.,electrolyte containing dissolved metal salts which result from continueddissolution of consumable electrodes 6 and 6'. The product may bedirected to hold up tank 22 through line 21 to await further handling.Further valve means 20, 23, and. 25 and 26 are positioned at variouspoints of the apparatus whereby to afford complete control of fluid flowthroughout the system. Valves 23 and 26 in upper and lower electrolytebridges 3 and 4 respectively permit each half cell to be isolated shouldchemical analyses, i.e., quantitative, qualitative, etc. of the processstream be desired.

Operation of the aforedescribed apparatus, specific reference being madeagain to metals dissolution processing, may be effected in the followingmanner. The metal selected for dissolution treatment is introduced intoeach of half cells 1 and 2. Suitable means may be provided foraccomplishing this step in either batch or continuous fashion. Thus, inthose instances wherein the metal to be treated is provided in bulkform, a suitable feeding hopper (not shown) may be positioned adjacenteach of the half cells to enable metered feeding. Alternatively, the rawmaterial metal may be provided insolution-form in which case feeding maybeaccomplished via a suitable conduit line (-not shown).provided withvalve means. ln anyevent, the manner in which metal feeding isaccomplished is not a particularly critical factor in the practice ofthe present invention, depending solely upon the requirements of theprocessor, e.g., whether continuous or batch operation is desired.

. The metal thus introduced comes to rest in contact system throughvalve 18 to an extent sufficient to fill each of the half cells as wellas the appertaining conduit lines. The acid materials may be any ofthose conventionally employed in the metal processing industries foraccomplishing electrolytic dissolution of raw material metals, withtypical representatives including, without necessary limitation,sulfuric acid, hydrochloric acid,

etc; in some instances the use of organic acids may be dictated such ascitric acid and the like. The particular acids selected will of coursedepend primarily on the dissolution rates desired by the processor. Theconcentration of the acid solution may vary ranging from about 10 toabout 80% by volume with a range of about -30 percent being particularlypreferred. In general, it is found that the use of acid solutions havinga pH ranging from about .1 to 2 are admirably suited to the purposes ofthe present invention. Vent valves 24 and 28 which remain openthroughout the acid introduction are at this point closed. The metaldissolution reaction rate depends of course, to a great extent upon thesusceptibility of the metal to acid attack, i.e., its reactivity whichin turn is influenced by the previous history of the metal, its extentof oxidation, corrosion and the like. I

The thermal gradient across half cells 1 and 2 essential to the practiceof the present invention is established by merely supplying heat to halfcell 1, hereinafter referred to as the hot half cell. This can beaccomplished by conventional means, i.e., most any of the systemspromulgated in the art for effecting efficient heat transfer. Forexample, the hot half cell may be provided with a heating jacket toprovide an annular space surrounding the cell which serves toaccommodate a heating fluid which may be in either vapor or liquid form.Alternatively, thermal energy may be supplied by means of an externalheat exchanger or by radiant energy transmission. In any event, theexact nature of the heating means adopted is significant only insofar asdesign considerations promotive of maximum energy utilization are deemedsignificant. As will be noted from the drawings, the mutual physicalarrangement of half cell 2 hereinafter referred to as the cold half celland the acid reservoir respectively may be such as to permit gravityfeed of the acid material into the system, i.e., by syphon effects. Suchan arrangement correspondingly reduces pumping requirements therebyminimizing the cost involved. The contacting of the acid medium with thestainless steel electrodes is accompanied practically simultaneously bythe initiation ofthe metal dissolution reaction. The net effect ofsupplying thermal energy to the hot half cell is the creation ofa'concentration'difference across the respectivehalf cells which canapparently be explained by reference to two mechanisms; firstly, theincreased metaldissolution reaction rate occurring in the hot half celldue to the reaction accelerating effects of the higher temperaturegenerates correspondingly, a higher concentration of dissolved metalsalts; secondly, the thermogradient established across the respectivehalf cells likewise gives rise to a further increment of concentrationgradient.

Each of the foregoing effects is operative in the creation of anelectromotive force and correspondingly the production of direct currentelectricity.

Considerable experimental investigation, study and evaluation suggeststhe conclusion that the manifold increase in metal dissolution reactionrate made possible by the present invention cannot be explained byreference to the harnessing of those particular energy valuesattributable solely to the-effects of temperature and concentrationdifferences. Without intending to be bound by any theory, it hasnevertheless been postulated in explanation of such phenomena that thefollowing situation obtains. Each of the thermal and concentrationgradients created across the respective half cells represents a sourceof free energy, the approximate value of either being capable ofmathematical resolution although somewhat approximate. In thisconnection, the Nerst theorum is deemed va1id.

However, the present invention is uniquely typified in that a stillfurther source of free energy results from the difference in therelative metal-dissolution reactions occurring in the half cells. Thiscan be explained asfollows: The reaction occurring in each of the halfcells is identical and can be explained by the following equations:

(cold half cell) M+HA R: M* +A H (hot half cell) M+HA B; M +A [-1wherein M represents the metal being treated, HA represents a suitableacid of the typementioned hereinbefore, M and A represent theproducts ofacid ionization and R and R connote' suitable indicia reflecting thereaction rate involved. v

As will be readily obvious R, is necessarily greater by a considerablemargin than R the disparity between becoming many times greater withincreased heat inputs to the hot half cells, i.e., as the temperaturegradient across the half cells is increased. A qualitative realizationof the extent of the differential in reaction rates can be appreciatedby reference to the rule of thumb" to the effect that the rate of achemical reactionis approximately doubled for each 10 rise intemperature. The difference in reaction rates which can be representedby A R gives rise to a further free energy source whose energycontribution to the metals dissolving sytemcan be at least assignificant as those energies derived from the concentration and thermalgradients. In fact, the energy provided by the AR value can exceedsignificantly the energy values attributable to the concentrationgradient, i.e., AC, and thermal gradient AT in view of theaforedescribed temperature-sensitivity of the reaction rate; otherwisestated, the temperature coefficient characterizing the mathematicalstatement defining the AR value would be relatively large and thusrelatively slight changes in temperature would have considerable impacton the respective half cell reaction rate and thus the'AR. Thus, thetherinogalvanic electrolytic cells described herein utilize 3 variablesi.e., temperature difference AT, concentration difference AC1 andreaction rate differences AR to achieve useful ends.

It is important to note at this point, that the desired metaldissolution reaction occurs in each of the half cells, i.e., oxidationoccurs at each of the electrodes. Within the context of the presentinvention, the hot half cell described herein can be regarded as ananode equivalent due to the fact that the electromotive force generatedthereby is positive relative to the cold half cell in the sense that amore accelerated metal dissolution reaction rate occurs thereat. Thisparticular observation has been confirmed gravimetrically by weight lossmeasurements of the metal in each half cell. Thus, the metal beingtreated, i.e., each of the consumable electrodes, is simultaneouslyundergoing dissoultion. I

The significance of this particular aspect of the present inventioncannot be emphasized too strongly. Heretofore, the energies diverted touseful purposes in the operation of metal-dissolving electrolytic cellswere in large part derived from competing reactions, i.e., oxidation andreduction respectively, one of the electrode members being of the inerttype. With respect to metals dissolution, such methods possess theobvious and inherent limitation that purposive ends, i.e., thesolubilization of metal, are achieved at one electrode only. The presentinvention thusprovides what must be considered a vital advance in theart in that the necessity for the use of competing reactions, i.e.,redox, in the operation of electrolytic type cells for purposes ofaccumulating energies is completely obviated. Although the basicoperative principle characterizing the mechanism of oxidation-reductioncells is involved, i.e., to the extent that a source of e.m.f. isestablished by virtue of an electrical difference in potential acrossthe respective half cells, the anode-cathode relationship results solelyfrom a simultaneous occurrence of identical reactions albeit atdifferent rates.

Optimization of the'improvement described herein are obtained bymaintaining hot half cell temperature within the range of from about50C. to about 70 C. and preferably from 58 C. to 65 C. During actual celloperation, valves 24 and 28 in vent lines 29 and 30 remain closed, whilevalves 25and 26 remain open in order to permit gaseous electrolysisproducts to be expelled into gas collecting tank 11. Although suitablepumping means may be employed if desired for purposes of accelerating orotherwise expediting the flow of fluid throughout the system, any suchmeans will usually not be required for most applications. Actually, thecombined effects of heat input to the hot half cell, the heat lossexperienced via radiation and convection (whether natural or induced)from the cold half cell serve to promote a density gradient across thehalf cells, such density gradient providing the driving means forinitiating as well as sustaining the necessary fluid flow volecity; thedirectional flow of fluid occurring in an upwardly direction through thehot half cell through the upper electrolyte bridge downward through thecold half cell and thereafter to the bottom of the hot half cell throughlower electrolyte bridge 4. The continuous convection circulation ofelectrolyte aids considerably in minimizing the possibility of cellpolarization which might otherwise occur and thusto this extent en'-hances the metal-electrolyte rate of reaction.- Other means may beresorted to whereby to establish some degree of fluid turbulence in theimmediate vicinity of the consumable .metal electrodes-in order tocombat cides with that point in the processing corresponding to 7 totalconsumable electrode. Such terminal point can be detected according to anumber of methods, e.g., colorimetrically, increase in electrolytedensity, chemical analysis-qualitative and/or quantitative, decrease inthe weight of metal added initially, etc. Continuous processing may beeffected, of course, simply by introducing the metal to be treated intoeach half cell on a periodic basis, as previously described. Theprescribed intervals maybe readily determined by correlating the rate ofmetal dissolution with the mass of metal to be treated.

' The salt' solution obtained as a result of metal dissolution ispreferably withdrawn continuously from the system with suitable valvemeans being provided for such purposes as indicatedat 19. Thetemperature of the product solution withdrawn from upper electrodebridge 3 will be lower than the temperature prevailing in the hot halfcell. Such solution is then directed through conduit means 21 intoproduct storage tank 22 and allowed to cool. Crystallization of solidsalts will accompany solution cooling. Supernatant liquor i.e.,saturated salt solution may be re-cycled to cold half cell 2. Saltrecovery may be effected by several techniques. Quite naturally, somesalt will precipitate on cooling i.e., crystallization. With mixedsalts, such as would be the case when treating alloys, separativerecovery is best effected by redissolving such salts and adding specificchemicals to preferentially precipitate the respective metals. Othertechniques'which may be resorted to V ment is given solely for purposesof exemplifying a specific arrangement of parts found to be convenientin practicing the present invention. Regardless of the particular formofapparatus employed, the essential units comprise, respectively, thehot and cold half cells, a first electrolyte bridge interconnecting theupper portionsof each of said half cells, a second electrolyte bridgeinterconnecting the lower portions of said half cells, andinertelectrodes-disposed within each half cell. The foregoing elementscomprise the essential components ofthe apparatus described herein. Suchitems as gas-collecting tank acid reservoir tank, etc., can be regardedas being auxiliary in nature and merely provide convenient meansforcarrying out themetals dissolution processing. During actual celloperation of prises a highly effective source of hydrogen gas,i.e.,

due to the accelerated electrolysis reaction involved as well as thehighly efficient utilization of the energies generated, the cost ofhydrogen gas generation can be reduced to an extent never beforerealized. This aspect of the present invention isof the first order ofcommercial significance in view of the fact that hydrogen is a highlyvaluable raw material for many industrial processes; thus, its economicproduction for use in conventional fuel cells continues to remain one ofthe unsettled problems of modern technology. The present thermogalvaniccells are likewise extremely useful as a test device whereby todetermine rates of galvanic corrosion. This affords ready means fordetermining the applicability of a given metallic substance to useswhich involve significant risks of corrosion. Another highly importantutility characterizing the thermogalvanic cells described herein relatesto their use as generators of electricity. As previously described,since cell operation proceeds as a result of the respective energycontributions attributable to differences in concentration, temperature,and reaction rates, and since each of such energy contributions isoperative in the formation of an emf, the electrical energy realizablethereby greatly exceeds the corresponding energy outputs of priordevices proposed for similar purposes.

Nevertheless, the utilization of the aforedescribed energy quantities tometals dissolution processing has provided a particularly fruitful areaof utility.

The following examples are given for purposes of illustration only andare not to be regarded as necessarily constituting a limitation on thescope of the present invention.

In each of the examples which follow, the apparatus employed wasComprised of the following arrangement of parts:

Hot Half Cell: 6 inch glass test tube, circular crosssection, 2 inchdiameter having open ends, each of said ends fitted with a 3-hold rubberstopper and a 2-hole rubber stopper, respectively;

Cold Half Cell: same as hot half cell;

Acid Reservoir: 1,000 ml. glass flask fitted with 2-hole rubber stopper;

Gas Collecting Bottle-trap: one liter flask open at one end and fittedwith a 2-hole rubber stopper.

The above described component parts are interconnected in the followingmanner. Each of the half cells is disposed vertically with a 3-holestopper end facing upward. The glass tube serving as the cold half cellis disposed approximately 8 inches higher than the glass tube serving asthe hot half cell. This arrangement is preferred since it permits theutilization of siphon effects whencharging the apparatus with the acidicelectrolyte. The upper ends of each of half cells are provided with aninterconnecting glass tubing approximately inches in length (V4 inchdiameter), such line serving as the upper electrolyte bridge. The endportion of such electrolyte bridge in the immediate vicinity of the coldhalf cell is fitted with a T-joint which connects respectively with (l)the cold half cell, (2) the upper bridge line, and (3) the acidreservoir flask, the latter through a length of glass tubing measuringapproximately 48 inches in length (V4 in diameter) and provided with apolypropylene stopcock to permit control over acid flow into the system.The end portion of the upper electrolyte bridge in the immediatevicinity of the other half cell, is likewise provided with a T-jointprovided with a polypropylene stopcock, (2) the hot half cell, and (3)the upper electrolyte bridge. The cold half cell is further providedwith a length of glass tubing measuring approximately 40 inches inlength (V4 inch diameter) which connects with the gas collecting bottleor trap disposed vertically above each of the half cells. This lineserves as a conduit for transporting effluent gases evolved during theelectrolysis reaction to the trap. The upper end of the hot half cellconnects with this line through a lingth of glass tubing approximately22 inches in length (V4 inch diameter). Each of the upper ends of thehalf cells is provided with a thermometer whereby necessary temperaturemeasurement may be taken. The lower ends of the half cells areinterconnected in the following manner. A length of glass tubing (V4inch diameter) disposed horizontally beneath the half cells and servingas the lower electrolyte bridge and measuring approximately 15 inches inlength is joined to the hot half cell by a vertical length of glasstubing extending upwardly and measuring approximately 2% inches inlength (V4 inch diameter) and to the cold half cell by a vertical lengthof glass tubing extending upwardly and measuring approximately 1 1inches in length (V4 inch diameter). The lower electrolyte bridge lineis provided with a polypropylene stopcock in the line connecting theaforedescribed vertical tube lengths, as well as in each of its endportions extending outwardly of each of the vertical leg connectingpoints. The latter valves permit withdrawal of electrolyte solution fromthe system as desired. Each of the bottom ends of the half cells isfurther provided with a carbon electrode of substantially circularcross-section measuring 2% inches in length and 5/16 inch in diameter.Each of the electrodes protrudes approximately V2 inchinto the internalportions of the half cells. The electrodes are electrically connectedthrough a length of copper wire measuring approximately 47 inches inlength and coupled through a single-pole-single-throw (SPST) switchwhich permits a voltmeter or ammeter to be inserted into the line tothereby permit the desired current or e.m.f. measurements. Each of thevoltmeter (0.3 volts) and ammeter (0-500 milliamps) is provided with adouble-pole-single-throw (DPST) switch which allows electricalcontinuity to be established as'desired thereacross. The hot half cellis immersed in a water-filled aluminum tank equipped with an immersionheater (1,000 watts).

EXAMPLE 1 chromium 18% nickel 8% manganese 2% iron 72% Electricalcontinuity across the carbon electrodes is established by closing theSPST switch. At this stage of the operations, the vent line connected tothe cold half cell as well as the intermediate stopcock valve in thelower electrolytic bridge are open. Sufficient acid from the acidreservoir flask is charged to the system whereby to fill each of thehalf cells and the upper and lower electrolytic bridge lines. The acidemployed in this example comprises aqueous sulfuric acid (10 percent H50 by volume) having a pH between 1 and 2. The temperature of the'hothalf cell is raised to approximately 28 C. by heating the water in whichit is immersed. At this temperature, the stainless steel dissolutionreaction accelerated visibly. The reaction was allowed to proceedunabated for a period of 3 hours with voltmeter and ammeter readingsbeing taken periodically.

EXAMPLES 2 and 3 Example I is repeated except that the concentration ofsulfuric acid is increased to 20 percent 58 10 percent by volumerespectively. The results obtained are summarized in the followingtable:

TAB LE I sitating resort to the use of special, high-pressure equipment,It is further quite possible that adjustments in the composition of theelectrolyte may be necessary, e.g., the use of additional ingredients tosurpress vapor pressure. In any event, the salient point to be notedconcerns the fact that the reduced temperature pro- Wgt. stainless steelAs the data in the above table clearly indicates, acid concentrations,by volume, approximating 20-30 percent provide exceptionally high metalsdissolution rates thereby enabling the dissolution of approximately 70percent by weight of the original metal charge in a period of only 3hours. The significance of this data can be made readily manifest bycomparison with the results obtained utilizing more standardizedtechniques previously resorted to for effecting metals dissolution. Forexample, experimental runs'based on simple acid immersion were conductedemploying identical stainless steel samples and sulfuric acid at varyingconcentrations as the electrolyte medium. In each case, the period ofimmersion in the acid was in excess of 3 hours. The results obtained aresummarized in the following table.

TABLE [1 Acid Concentration Wt. Stainless Steel (grams) %(Vol InitialDissolved l0 1.05 0.01 20 1.02 0.0] 40 1.02 0.01 8 0 1.1 1 0.01

The dissolution rates obtainable with simple immersion techniques asclearly indicated in Table II above are intolerably low and wouldscarcely suffice for commercial requirements. The use of elevated acidtemperatures provide some degree of improvement however, of vitalimportance is the fact that in each .case, utilizing acidconcentrations, by volume of 10 percent, percent, 40 percent and 80percent, respectively,

t mp atu es in. exc spflfiqflwe necess n.

order to obtain metals dissolution reaction rates which would even beginto compare with those typifying the process and apparatus of the presentinvention. This is to be contrasted with the temperature valuescharacterizing operation of the subject cell in the foregoing examples,i.e., on the order of 55 C. to 63 C. The extremely high temperaturesrequired for feasible practice of prior art techniques such as typifiedby the aforedescribed acid immersion treatement make mandatory the useof highly costly, temperature-resistant materials of constructionspecifically adapted to withstand the corrosive nature of hightemperature acidic media. As a matter of pure economics, the costinvolved can be prohibitive. Furthermore, since the operatingtemperatures employed would of necessity exceed 100 C. and thus thenormal boiling point of water, the maintenance of constant electrolytesolution concentrations can be somewhat problematical. Operation athigher pressure may well be dictated thus necescessing characterizingthe present invention greatly increases the operators scope ofoperations.

The product solution obtained in the foregoing examples upon completionof dissolution operation was determined by analysis to comprise amixture of metallic sulfates, i.e., CrSO FeSO NiSO and MnSO Aspreviously described, separation of the salt mixture into its variouscomponents may be readily and easily accomplished by, for example,re-dissolution followed by preferential precipitation. Alternatively,fractional crystallization or resin bed absorption may be resorted tosuch purposes.

Results similar to'those described above are obtained when theprocedures exemplified are repeated by employing in lieu of stainlesssteel equivalent quantities of 10,10 scrap, iron, zinc, etc.

It will also be observed from the foregoing examples that optimum metalsdissolution reaction rates are obtained with the use of acidconcentrations ranging from 20-30 percent by volume. This can probablybe explained by reference to the fact that the degree of aciddissociation increases with increased dilution within, of course,certain limits. With acid solutions of high dilution, i.e., on theorder" of 10 percent by volume, solution activity is apparentlydecreased to the extent that the degree of acid dissociation becomesrelatively insignificant. In any event, the aforestated acidconcentrations, namely, 20-30 percent by volume, are found tobe'admirably suited to the purposesof the present invention andparticularly within the temperature ranges extant during actual celloperation.

The various container means comprising the several components of theapparatus described herein as well as the conduit lines, valves, etc.must of course, be resistant to acidic solutions having a pH as lowas 1. A wide variety of materials are readily available commerically inthis regard. However, particularly beneficial results are obtained withmany of the synthetic, organic plastic materials currently on themarket, e.g., polyethylene, Materials of this type possess excellentstructural ability, are substantially inert i.e., in no waydeleteriously affect the electrolyte solution, while exhibitingexceptional resistance to/the corrosive effects of the acidicelectrolyte. Such materials are, in addition, desirable from an economicstandpoint since they are relatively inexpensive both in cost andmaintenance.

The sizes of the various apparatus components will depend primarily onthe volume requirements of the processor as well as the flowcharacteristics of the process stream. Such considerations are notparticularly critical factors in the practice of the present inventionand are readily capable of determination byrather routine means. Forexample, it will usually be advisable to maintain linear as opposed toturbulent flow in the connecting conduit lines, this being moreconducive to efficient transport of the ionic species between therespective half cells.

This invention has been described with respect to certain preferredembodiments and there will become obvious to persons skilled in the artother variations, modifications, and equivalents which are to beunderstood as coming within the scope of the present invention.

What is claimed is:

l. A process for effecting the rapid dissolution of metals in an acidsolution which comprises placing the metal to be treated in contact witha first inert unreactive electrode and a second inert unreactiveelectrode, each of said electrodes being situated in physically separatefirst and second container means, said container means beinginterconnected at their upper regions by a first conduit means servingas an upper electrolyte bridge and at their lower regions by a secondconduit means serving as a lower electrolyte bridge whereby to provide acontinuous flow path between said container means, said first containermeans being maintained at a higher temperature than said secondcontainer means the concentration of acid being from about 10 to 80percent by volume.

2. A process according to claim 1 wherein said acid solution comprisessulfuric acid.

3. A process according to claim 1 wherein said metal comprises stainlesssteel.

4. A process according to claim 1 wherein the acid is at least onemember selected from the group consisting of sulfuric acid, hydrochloricacid and organic acids.

5. A process according to claim 1 wherein the pH of said acid solutionis from about 1 to about 2.

6. A process according to claim 1 wherein the temperature maintained insaid first container means is from about 50 C. to about C.

7. A process according to claim 6 wherein the temperature maintained insaid first container means is from 58 C. to 65 C.

8. A process according to claim 1 wherein the concentration of acid isfrom 20 to 30 percent by volume.

2. A process according to claim 1 wherein said acid solution comprisessulfuric acid.
 3. A process according to claim 1 wherein said metalcomprises stainless steel.
 4. A process according to claim 1 wherein theacid is at least one member selected from the group consisting ofsulfuric acid, hydrochloric acid and organic acids.
 5. A processaccording to claim 1 wherein the pH of said acid solution is from about1 to about
 2. 6. A process according to claim 1 wherein the temperaturemaintained in said first container means is from about 50* C. to about70* C.
 7. A process according to claim 6 wherein the temperaturemaintained in said first container means is from 58* C. to 65* C.
 8. Aprocess according to claim 1 wherein the concentration of acid is from20 to 30 percent by volume.